The long range purpose of this project is to investigate the role of type C retroviruses as an etiologic agent and a vector of genetic information for neoplasia, and the immunologic responses of the host to tumor associated antigens. The topics of current interest are: 1) Further study of the role of cloned B-tropic viruses (11A-MuLV) in transmitting transplantation immunity against Meth A tumor. 2) Characterization of newer isolates of xenotropic, dual-tropic, and amphotropic viruses by competitive radioimmunoassays. 3) Further studies on the interferon and natural killer (NK) activity associated with transplantable and spontaneous reticulum cell neoplasm (RCN) of SJL/J mice. 4) Immuno-potentiating effect of a glucan (Lentinan). 5) Effects of interferon and tumor promoter on the NK-sensitivity of trophoblast cells.